1. Field of the Invention
In general, the present invention relates to a process for purifying carbon blacks. More specifically it relates to heat treating carbon blacks to reduce the levels of polycyclic aromatic hydrocarbons on the surface of the carbon black.
2. Description of the Prior Art
Carbon blacks contain a low level of polynuclear aromatic hydrocarbons (PNA) on their surface which are believed to be carcinogenic. Because of this, the FDA has restricted the use of carbon blacks in applications which involve contact with food or skin (21 CFR S121.2562). Industry has tried to reduce these PNA levels by modifying the carbon black process by using high temperatures in the furnace and delaying the quench. This reduces the PNA levels from a range of 100 to 150 ppm down to levels of 25 to 40 ppm. The present invention brings down the PNA levels to below 2 ppb.
The concept of purifying carbon black is old in the art. In U.S. Pat. No. 1,303,362, Mott purifies lampblack by heating it in the presence of carbon tetrachloride at temperatures of 1472-1832.degree. F.
In U.S. Pat. No. 2,643,182 carbon black is purified by Williams by suspending the carbon black in hot combustion gases and bringing the temperature up to 1562-3632.degree. F for 0.1 - 5.0 seconds.
In British Pat. No. 937,841, the carbon black in a slurry is purified by mixing into the carbon black slurry a volatile liquid which is immiscible with water and has a higher adhesion tension than water for the carbon black. The water is then drained off, and the volatile liquid is evaporated off, leaving a purer carbon black.
Williams uses a dilute solution of NH.sub.4 NO.sub.3 in U.S. Pat. No. 3,512,935 to purify carbon black.
A different technology has developed for reducing the pH of carbon black. The low pH values (2-5) observed for channel blacks arise from the carboxyls, phenols, lactones and other acidic oxides present on the surface. Furnace blacks have considerably higher pH values, generally in the range of 7-10, due to the absence of appreciable amounts of these acidic surface oxides.
Considerable interest has been generated in introducing acidic surface oxides onto the surface of furnace blacks to prepare "channel-like" materials. Such modifications have been accomplished by reacting furnace blacks with nitric acid, nitrous oxide, ozone, etc. and with oxygen or air at temperatures above ambient. The best pH reduction with air oxidation occurs in the temperature range from 400 - 750.degree. F. These temperatures are too low to prepare PNA-free blacks. The use of higher temperatures (above 750.degree. F) are not as successful at pH reduction due to the low thermal stability of the acidic oxides at these temperatures.
Cines (U.S. Pat. No. 2,682,448), oxidizes carbon black by exposing it in a tumbling drum to an oxygen-containing gas (21/2 - 5%) for 16 minutes to 2 hours, at temperatures of 400 - 1200.degree. F.
Sweitzer (U.S. Pat. No. 2,707,674), oxidizes carbon black in a shallow bed for 50 minutes at 650 - 950.degree. F.
A fluidized bed is used by Pollock in U.S. Pat. No. 3,247,003 to oxidize the carbon black. Ozone is used as the oxidizing agent at temperatures of 650 - 1000.degree. F for 1 hour. (The use of fluidized beds for processes dealing with carbonous material is outlined by Odell in U.S. Pat. No. 1,984,380.).
Daniell and Peterson (U.S. Pat. No. 3,279,935), use peroxide with air to oxidize carbon black in a fluidized bed maintained at 350 - 600.degree. F.
Johnson, Logan and Larson (U.S. Pat. No. 3,318,720), impregnate carbon black with a compound such as hydroxide, nitrates, etc., prior to air oxidation in a fluidized bed at a temperature of 550 - 950.degree. F.
In U.S. Pat. No. 2,714,055, Cines used the technology developed for reducing pH in U.S. Pat. Nos. 2,682,448 and 2,707,674 to remove tar from carbon black at temperatures of 400.degree. F to 1200.degree. F.
In U.S. Pat. No. 3,411,928 Dollinger and Joy oxidize furnace black by air in a fluidized bed at 550 - 825.degree. F for 30 minutes to 21 hours to make it like channel black.
Activated carbons do not have high PNA levels, but these carbons are more expensive and have high surface area, high porosity, high moisture absorption which gives problems in compounding rubber or plastic. Also, if the activated carbon is made from coal, it does not have the reinforcing properties, or the coloring properties and lacks the purity in inorganic metals, ash and inorganic salts that regular carbon black has.
None of the prior art has been successful in making a carbon black with the proper coloring properties and reinforcing properties that has PNA levels below the ppm range.